Here's a place where I can post my thoughts on new papers, provide updates on my projects, and post info that will eventually be on my website The Theropod Database - http://theropoddatabase.com/ . It will center on theropods, but may delve into other topics as well such as phylogenetics.

Wednesday, May 4, 2011

Nesbitt's excellent new analysis

I just wanted to state my praise for Sterling Nesbitt and his amazing new archosaurian phylogeny paper. The taxon sampling is extensive, with discussions of which material is used for each taxon. The characters are described in great detail and illustrated wonderfully, with the specimens they're found in often listed. Alternative phylogenies are examined and character support is discussed. The matrix seems completely coded. I chuckled at the dismissals of Lucas' "superficial" taxonomic discussions. :) This is surely one of the best archosaurian analyses published, and that includes ALL archosaurian analyses, on theropods, birds, crocs, you name it. But it just wouldn't be like me to not include some criticisms, so I offer the following...

On page 236, Nesbitt discusses the position of pterosaurs, which is as basal avemetatarsalians. Note this doesn't mean much in regard to the controversy of their wider relationships among reptiles, since simiosaurs, Longisquama and tanystropheids were not included. Nesbitt tries to counter Bennett's (1996) statement that pterosaurs don't have many synapomorphies of archosauriform clades more inclusive than Ornithodira with a list of the synapomorphies of Crurotarsi sensu lato, Archosauria, Archosauriformes, etc., detailing which are found in pterosaurs. This is all fine except that 13 of the characters are simply stated to be "not known in basal pterosaurs". I can't help but wondering if these (generally difficult to observe features in the braincase, tarsals and such) are known to be absent in derived pterosaurs. If so, they should still count against pterosaurs being in the listed clades. In fact, I wonder why no derived pterosaur like Pteranodon was included, when derived members of other clades (e.g. Alligator, Allosaurus, Velociraptor) were used to increase the informativeness of characters.

Note Nesbitt's Rauisuchidae should be called Teratosauridae, assuming Teratosaurus is in its standard position by Polonosuchus and Postosuchus.

I would have liked to see Doswellia, Trialestes, "Zanclodon" arenaceus, Erpetosuchus (oddly included in the character descriptions, but not the matrix), Hallopus and/or Macelognathus, Scleromochlus and Preondactylus included.

On page 187, Nesbitt is right to note that many analyses have "Characters just listed with no, little, or
vague explanations". How reduced is "reduced"? How tapered is "tapered"? Unfortunately, Nesbitt's characters sometimes have the same problem. For instance, character 282 is "Pubis, length: (0) shorter or subequal to the ischium; (1) longer than ischium." The word "subequal" should be expunged from character descriptions, since it's not quantified. If I have a taxon with a puboischial ratio of 101%, that's presumably subequal. But what if it's 103%? 106%? Everyone's conception will be different, and this will lead to miscodings. Or character 259- "Metacarpal IV: (0) present; (1) reduced to a nubbin or absent." When is a metacarpal a "nubbin"? Or character 266- "Ilium, crest dorsal to the supraacetabular crest/rim: (0) vertical; (1) anterodorsally inclined." Vertical compared to what basis of horizontal (sacrum axis, axis between anterior and posterior tips, axis formed by ventral extent of peduncles, etc.), and would 92 degrees count as vertical or inclined? These kinds of issues are found in almost every analysis, so this isn't a critique of Nesbitt as much as it's an urging to all of us to explicitly define our character states.

But these issues aside, the paper is extremely well done. Besides the above taxa, someone should use this matrix to place Arganasuchus, "Cryptoraptor", Lagosuchus, Lukousaurus, Razanandrongobe, Saltopus, Spondylosoma, Stagonosuchus, Yarasuchus and Zanclodon.

Nesbitt, 2011. The early evolution of archosaurs: Relationships and the origin of major clades. Bulletin of the American Museum of Natural History. 352, 292 pp.

Unfortunately, my copy of the Dinosaur Society encyclopedia is at my parents', so I can't check if "Dinosaurus"' supposed size was mentioned. But the fragmentary ribs wouldn't agree with Dryptosauroides, which already had an entry if I recall correctly.

As for Mangahouanga, it's a stream and the fossil site around it which have yielded dinosaur bones, including those of theropods. I wouldn't be surprised if it's just a misunderstanding of some statement like "The Mangahouanga vertebrae...".

Late Cretaceous 88.5 to 83 millionIndiaMeatThis carnivore is known only from large tail vertebra and some fragmentary ribs. The size shape of vertebra -13 to 14 centimeters (about 5 inches) long.- indicates that Dinosaurus was large and may have been heavily built. The vertebra does not have the notches that, in some dinosaurs, lessened the animals´ weight.

References

Glut/Lessem 1993. Dinosaur encyclopedia.

Dryptosauroides grandis

Late Cretaceous 97.5 to 65 millionIndiaMeat

(meaning 'similar in form to Dryptosaurus') is the name given to a genus of dinosaur from the Late Cretaceous. It was a large theropod possibly belonging to the Abelisauridae.

Its fossils, consisting of six caudal vertebrae, together forming type specimens GSI IM K20/334, 609, K27/549, 601, 602 and 626, were found in India in the Lameta Formation of the Maastrichtian. The vertebrae, originally falsely identified as dorsals, are thirteen to fourteen centimetres long. These remains are today commonly considered to be indistinguishable from those of other theropods from the same formation. As a result, Dryptosauroides is seen as a nomen dubium.

The type species, Dryptosauroides grandis, was named by Friedrich von Huene in 1932 and described by von Huene and Charles Alfred Matley in 1933. The specific name means "large" in Latin.

Davidow- Zanclodon was eventually redescribed, by Galton (2001). Should be easy to add since it's just a partial maxilla, but with interesting characters like no interdental plates or tooth serrations.

Good question. I've heard both rauisuchian and herrerasaurid, but the former was at least in part due to the ventrally angled sacral ribs, which is only found in Erythrosuchus and some suchians in Nesbitt's matrix.